1. Field of the Invention
The present invention relates generally to a lens apparatus for projection wherein the adjustment of contrast and shading means are improved.
2. Description of the Prior Art
In recent years, television projectors are widely utilized as displays for displaying images received from television broadcasting, as well as images outputted from electronic image apparatuses, such as VTRs. Although lens apparatuses for projection for use in such projectors have a very great power, it is naturally required that the lens apparatuses have various small aberrations to realize good optical characteristics.
An example of such a lens apparatus for projection is shown in FIG. 2. This lens apparatus for projection 1 comprises a negative first lens group 2, a positive second lens group 3, a positive third lens group 4 having the strongest power of component lenses, a positive fourth lens group 5, a positive fifth lens group 6, a negative sixth lens group 7, and a negative field curvature correction unit 8 for correcting almost all of the curvature of field of the lens system, which are arranged in that order from the side of a screen. In this example, each of the first through sixth lens groups is a single lens. This lens apparatus for projection 1 is integrated with a cathode-ray tube (not shown) to be used.
The field curvature correction unit 8 comprises a left first lens portion 8A, a right second lens portion 8B, a cooling liquid 8c, which is filled therebetween for fulfilling lens functions, and an inked portion 8D which is provided on the left side face of the first lens portion 8A. The inked portion 8D is applied on the periphery of the left side face of the first lens portion 8A for removing undesired light, such as scattered light, which is incident on the periphery of the lens, to prevent light from diffusing.
In accordance with colors of RGB, three lens apparatuses for projection are provided in parallel. These three lens apparatuses for projection for colors of RGB are usually arranged laterally.
By the way, in a system for projecting light from three light sources for RGB, which are laterally arranged, to project an image, there is the following disadvantage. That is, since the central light source is provided at a position, at which its optical axis is perpendicular to the surface of the screen, to perpendicularly irradiate the surface of the screen with light, the difference in contrast does not appear on the irradiated surface, but the difference in contrast appears by the light sources which are provided on the right and left sides. That is, light emitted from the light source provided on the left side with respect to the screen is bright on the surface of the screen on the left side which is near the light source, and dark on the surface of the screen on the right side which is distant from the light source, whereas light emitted from the light source provided on the right side with respect to the screen is bright on the surface of the screen on the right side which is near the light source, and dark on the surface of the screen on the left side which is distant from the light source.
In order to solve this disadvantage, a contrast adjusting plate (not shown) is provided on the left side of the first lens portion 8A of the field curvature correction unit 8. This contrast adjusting plate comprises a single member, such as an elongated plate, and is arranged on the left side of the first lens portion 8A. Since the contrast adjusting plate must be accurately positioned, a portion for supporting the contrast adjusting plate is provided.
However, the operation for applying the above described inked portion 8D on the periphery of the left side face of the first lens portion 8A can not be easily carried out, and one step of applying operation is added to the assembly operation.
In addition, the lens apparatus for projection must be provided with the portion for supporting the contrast adjusting plate, so that the structure of the lens apparatus for projection is complicated.
As a result, there is a problem in that the cost of producing the lens apparatus for projection is increased.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a lens apparatus for projection, which has a simple structure and which can be easily assembled to reduce the producing costs.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a lens apparatus for projection comprises: a lens or two or more lenses provided on an optical axis; and a shading plate, provided as a single member on the optical axis with the lens or one of the lenses, for covering a peripheral portion of the lens or the one of the lenses to block off light entering the peripheral portion.
With this construction, since the shading plate is provided as a single member, it is possible to omit an inking process, which takes time.
The lens apparatus for projection may further comprise a contrast compensating plate for compensating contrast of a projected image, the contrast compensating plate being integrated with the shading plate.
With this construction, since the contrast compensating plate is integrated with the shading plate, it is not required to provide any portions for supporting the contrast compensating plate, so that it is possible to reduce the number of parts. Thus, it is possible to reduce the costs of producing the lens apparatus for projection since it is also possible to omit the inking process which takes time.
The lens apparatus for projection may further comprise a shading wall for blocking off light leaking out of the peripheral portion of the lens or the one of the lenses, the shading wall being formed on a peripheral portion of the shading plate by extending or bending the peripheral portion of the shading plate.
With this construction, the shading wall can surely block off light, such as scattered light, which leaks out of the periphery of the lens.